1. Field of the Invention
The present invention relates to the development and synthesis of divalent europium activated alkaline earth halide aluminate phosphor particles having a uniform particle size distribution (0.5-5 xcexcm) and a method of forming such particles. More particularly, the present invention relates to the formation of such particles from respective oxides, carbonates, and nitrates in presence of activated charcoal. The phosphor of present invention exhibits improved stability and longer life when used in a plasma display panel.
2. Description of the Prior Art
Divalent europium (Eu2+) activated barium magnesium aluminate (BAM) phosphor, is currently used in plasma display panels (PDP) as a blue emitting component. When compared with other phosphors used for red and green, BAM phosphor exhibits lower life or less stability when exposed to vacuum ultraviolet (VUV) flux in a PDP. The maximum wavelength from blue phosphor also shifts towards green during the various process steps in display manufacturing. This shift is called a xe2x80x9cgreen shiftxe2x80x9d.
U.S. Pat. Nos. 5,989,454 and 6,187,225 describe the continued efforts that are being made to improve the existing BAM (BaMgAl10O17:Eu2+) phosphors and develop new phosphors such as LaPO4:Tm3+ to address this problem and satisfy other requirements.
U.S. Pat. No. 3,294,699 discloses the invention of BAM as a blue phosphor. According to U.S. Pat. No. 4,110,660, the blend containing BaF2, LiF, Al(OH)3 and Eu2O3 is fired in a H2 atmosphere in the temperature range of 1400xc2x0 C.-1650xc2x0 C. for a period of 3 to 6 hours to obtain blue emitting divalent europium activated barium-lithium aluminate phosphor, which can be used in xerography lamps.
In U.S. Pat. No. 4,161,457, Koichi Takahashi et al. have proposed a BAM phosphor as a high radiation blue emitting phosphor under excitation by vacuum ultraviolet rays having a wavelength shorter than 200 nm. This BAM was prepared by calcinating respective metal salts, such as, carbonates, nitrates, sulfates and halides, at temperatures in the range 1200xc2x0 C.-1600xc2x0 C. in two different firing cycles.
U.S. Pat. No. 4,249,108 describes a multi-phase structured Eu2+ activated La, Mg aluminate phosphor, which exhibited an improved brightness over prior art single-phase compositions. Starting materials (La2O3, MgO, Al(OH)3 and Eu2O3) were fired at 1500xc2x0 C.-1650xc2x0 C. for 1-5 hours in a reducing atmosphere (75 vol. % H2, 25 vol. % of N2). Other related art also reveals that Ba (up to 25%) can be replaced by Sr (see, for example, U.S. Pat. No. 4,590,405).
In U.S. Pat. No. 5,611,959, Kijima et al. describe the synthesis of an aluminate phosphor including (a) at least one element selected from the group consisting of Ba, Sr and Ca, (b) Eu, (c) Mg and/or Zn, (d) optionally Mn and (e) Al, obtained by firing the respective oxides and/or hydroxides in a reducing atmosphere at a temperature of from 1200xc2x0 C.-1700xc2x0 C. over a period of 2-40 hours.
A method of producing BAM without any fluoride compound (flux) in the starting ingredients was described in U.S. Pat. No. 5,879,586. According to this invention, the diameter (1-20 xcexcm) and the shape of the particles can be controlled by firing the samples in a reducing atmosphere at higher temperatures (1600xc2x0 C.-2000xc2x0 C.).
U.S. Pat. Nos. 5,989,454 and 6,187,225 B1 describe blue emitting phosphors.
European Patent No. 0 418 902 A2 describes the role of alkaline earth and zinc in europium activated BAM in three component phosphor blends in controlling fluorescent lamp performance.
According to Japanese Patent (8115673), BaMgAl10O17:Eu2+ has better time-wise durability when compared with traditional BaMgAl14O23:Eu2+. This particular phosphor has been synthesized by calcinating a barium compound, such as, BaO, Ba(OH)2, BaCO3, etc., an europium compound, such as, Eu2O3, EuF3, etc., a magnesium compound, such as, MgO, Mg(OH)3, Mg(CO3)4.Mg(OH)2.5H2O, etc., an aluminum compound, such as, Al2O3, Al(OH)3, etc., and a flux, such as, fluorides of Ba, Al or Mg. The calcinating is carried out at least once for 2-40 hours at 1200-1700xc2x0 C. in a reducing atmosphere in the presence of steam.
In WO 99/34389, Zachau has suggested the use of manganese in BAM as a co-activator along with europium to obtain better stability and maintenance of BAM in plasma display applications.
The main application area of large area plasma displays is High Definition Television (HDTV) and high information content presentation. HDTV and other similar types of display devices require phosphors with high resolution and high brightness for high performance. This can be achieved only with thin phosphor screens that have small phosphor particles in a close rib structure or closed cell structure. Screens with small particles exhibit higher packing density and also need reduced binder content.
Stability is another concern in selecting a phosphor. The degradation of phosphor should not be more than 10% before 10,000 hours of operation of a plasma display panel.
The three phosphors, i.e., red, green and blue, that are currently used in PDP""s, do not exhibit the same life, stability and particle morphology. Thus, the blue phosphor degrades very fast when compared with the green and red phosphors. During the display process, the color point, i.e., the color coordinates, of the blue phosphor (BAM) shifts towards green. This result compromises the performance of the display.
In view of these problems, our efforts have been directed to improvement of the existing phosphors (BAM) and the development new phosphors.
Accordingly, it is an objective of the present invention to provide development and a method of preparation of divalent europium activated alkaline earth halide aluminate phosphor having the empirical formula:
(AE1-xMgEux).(AH)y.Al10O17
wherein AE is an alkaline earth metal selected from the group consisting of: Ba, Sr and Ca;
wherein AH is an alkali metal halide or ammonium halide wherein the halide is Cl or F; and
wherein 0.01xe2x89xa6xxe2x89xa60.1 and 0xe2x89xa6yxe2x89xa60.1.
The present invention provides a method of producing a blue color emitting divalent europium activated alkaline earth halide aluminate phosphor having the empirical formula:
(AE1-xMgEux).(AH)y.Al10O17
wherein AE is an alkaline earth metal selected from the group consisting of: Ba, Sr and Ca;
wherein AH is an alkali metal halide or ammonium halide wherein the halide is Cl or F; and
wherein 0.01xe2x89xa6xxe2x89xa60.1 and 0xe2x89xa6yxe2x89xa60.1.
The method includes the steps of:
heating the chemical mixture including: a source of europium, a source of aluminum, a source of barium or calcium or strontium and a source of magnesium to 850xc2x0 C.; optionally
adding a source of a halide, such as, chloride or fluoride, to the mixture;
heating to 1000xc2x0 C. to 1600xc2x0 C. in the presence of air or an activated charcoal carbon; and
heating at 1000xc2x0 C. to 1400xc2x0 C. in presence of a reducing atmosphere forming gas, i.e., a 5% Hydrogen and 95% Nitrogen mixture, for a period of time sufficient to produce the phosphor.
The phosphors according to the present invention have a band emission in blue region, peaking around 450 nm when excited by 147 or 173 nm radiation from Xenon gas mixture. They have uniform particle size distribution (0.5-5 microns), which is appropriate for thin phosphor screens required for a variety of flat panel display such as plasma display panels and lamp applications. They also exhibit better stability or life when excited with VUV (147 and 173 nm) radiations from Xe discharge in a plasma display than the currently available commercial phosphors used in plasma display panels. The small size phosphor particles provided by the present invention are particularly suitable for use in high-resolution display applications.